1. Field of the Invention
The present invention generally relates to the technical field of mobile communications and particularly relates to mobile communications systems, base station apparatuses, user apparatuses, and methods that perform retransmission control.
2. Description of the Related Art
In general, schemes for implementing two-way communications include a full duplex scheme and half duplex scheme. A specific example of the former scheme includes a frequency division duplexing (FDD) scheme. A specific example of the latter scheme includes a time division duplexing (TDD) scheme. The FDD scheme, which use different frequencies for uplink and downlink, respectively, makes it possible to simultaneously communicate transmit and receive signals. The TDD scheme, which uses the same frequency for uplink and downlink, has uplink and downlink periods alternating. Currently, these schemes differ from country to country and/or from region to region.
As globalization moves forward, roaming between regions of different two-way communications schemes is also in the process of being performed. Roaming may be supported with relative ease when an FDD terminal being used in an FDD system enters a TDD region. This is because a transition from the full duplex scheme to the half duplex scheme is relatively easy.
On the other hand, immediate support for roaming when a TDD terminal being used in a TDD system enters the FDD system is impossible. This is because the TDD terminal cannot tune to two frequencies simultaneously to process signals in uplink and downlink separately. In this case, similar to the FDD terminal, it is possible to provide the TDD terminal with a duplexer and to arrange for uplink and downlink to process signals separately for uplink and downlink. However, such a method may cause the TDD terminal to become extremely expensive. From such a point of view, it is being considered to change a tuning frequency between TDD terminal uplink and downlink periods, and make the TDD terminal conduct half duplex communications as before, even within an FDD system. In this way, while benefits of full duplex communications may not be obtained, roaming of the TDD terminal to the FDD system may be supported in an easy and inexpensive manner. For example, Non-patent document 1 discloses conducting half-duplex communications while changing frequencies in the FDD system.    Non-patent document 1: 3GPP, R1-050731, Vodafone Group, T-Mobile International, TeliaSonera, Telefonica, “Support of operation in paired and unpaired spectrum”